A vast majority of human pathogens are known to initiate infections at mucosal surfaces, thus, making the gastrointestinal, urogenital and respiratory tracts major routes of entry into the body. As a result, the other primary way to contract an infection is through blood-borne routes such injections, transfusions and bites. Examples of mucosally-infecting agents include cold viruses, influenza, food poisoning agents tuberculosis, sexually transmitted diseases, cholera, diphtheria and the plague.
The mucous membranes are one of the largest organs of the body. Collectively, they cover a surface area of more than 400 m2 (equivalent to one and half tennis courts) and comprise the linings of the gastrointestinal, urogenital and respiratory tracts. These mucosal surfaces, while located inside the body, are actually a physical barrier between the outside and the sterile interior cavity of the body known as the “systemic” environment. Critical nutrients, oxygen and other molecules are constantly taken up across these mucosal barriers; however, another important function of the mucous is to keep invading pathogens out. Daily these mucous membranes are bombarded by outside elements and it is up to the unique immune system of the mucous to determine what is potentially harmful and what is beneficial.
The importance of mucosal immunology lies in the interplay between the mucosal response and the systemic immune response. Several studies have demonstrated that stimulating the immune system systemically (i.e. via injection or blood-borne routes) results in the production of protective antibody and T cells only within the sterile, internal environment of the body—no mucosal response is generated. On the other hand, stimulation of the mucosal immune response can result in production of protective B and T cells in both mucosal and systemic environments so that infections are stopped before they get into the body.
The mucous membranes produce a special type of antibody called secretory IgA or sIgA. The mucous membranes are bathed in huge quantities of sIgA, which act as a first line of defense to neutralize invading pathogens. Experimental evidence shows that the presence of sIgA correlates with resistance to infection by various pathogens, including bacteria, viruses, parasites and fungi. It has also been shown to neutralize viruses and prevent their adherence to the epithelial cells lining the mucous (thereby preventing infection) as well as mediating excretion of pathogens and preventing the assembly of mature virus particles.
Another important component of mucosal immunity is the T cell-mediated immune response. T-cells that specifically recognize pathogens can help antibodies to clear the infection or directly kill the invader themselves. T cells produced in the mucous are capable of traveling throughout the mucosal tissues through special “homing” receptors on their membranes. This means that if an immune response is generated in the gastrointestinal lining, T cells produced there can travel to other mucosal sites, for example, the lungs or nasal cavity, providing protection over a large area.
Despite the important role of the mucosal surface, only a handful of vaccines specifically target this area of the immune system, thus there remains a need for vaccines that are directed toward the mucosal surface to provide protective immune responses at the mucosal tissue.